Shiny Asteroid Vesta Once Had Magnetic Personality

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Vesta, the brightest asteroid in the solar system, apparently
possessed a magnetic field in its infancy that shielded it from
the ravages of energetic particles from the sun, researchers say.

The finding could help solve the mystery of why Vesta's surface
appears so bright, they add.

Vesta is the second-largest asteroid in the solar system, a
behemoth 330 miles wide (530 kilometers) that is sometimes
visible to the naked eye on Earth. The only larger asteroid is
Ceres, which is also classified as a dwarf planet.

Vesta occasionally experiences collisions, probably with other
members of the solar system's main asteroid belt, which lies
between the orbits of Mars and Jupiter. These impacts can knock
rocks off Vesta, with some of them plummeting to Earth as
meteorites.

For the new study, scientists analyzed samples from a shiny black
meteorite recovered in 1981 in hills at the end of the
Transantarctic Mountains in Antarctica. The rock's oxygen isotope
levels matched those astronomers have seen on Vesta, suggesting
it originated on that asteroid long ago.

Magnetized crystals within the meteorite suggest Vesta once had a
magnetic field strong enough to leave an imprint on its surface
rocks. Argon isotopes within the meteorite help pin down its age,
suggesting the asteroid's crust was still magnetized 3.69 billion
years ago. (If Vesta had a magnetic field, it probably died well
before that, once Vesta's core cooled and slowed.)

The researchers suggest Vesta had a spinning liquid metallic core
early in its history. This apparently generated a dynamo,
resulting in a magnetic field at least 2 microteslas in strength
and perhaps as strong as 10 to 100 microteslas. In comparison,
Earth's surface magnetic field is about 30 to 60 microteslas.

"Up to now, it was uncertain if small bodies like asteroids could
harbor a dynamo like that observed on larger planets such as
Earth," lead study author Roger Fu, a planetary scientist
at the Massachusetts Institute of Technology, told SPACE.com.
"It's good to be able to confirm models that said it was
possible."

The researchers think Vesta's ancient magnetic field may have
shielded the asteroid from scouring by winds of electrically
charged particles from the sun. This could help
explain why its surface is so bright — the solar wind normally
would darken the asteroid over time.

These findings could help researchers learn more about how the
building blocks of the rocky planets grew and evolved.

"Earth and the other terrestrial planets are made of objects like
Vesta," Fu said. "The coolest thing to me is that we're learning
more and more about planetesimals, about this critical stage in
the early solar system."

Fu and his colleagues plan on looking at ancient meteorites to
see if magnetic fields were present before even planetesimals
formed in the solar system's protoplanetary disk. They detail
their findings in the Oct. 12 issue of the journal Science.

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